Effects of Endotracheal Epinephrine on Pharmacokinetics and Survival in a Swine Pediatric Cardiac Arrest Model

OBJECTIVES

The aim of this study was to compare the endotracheal tube (ET) and intravenous (IV) administration of epinephrine relative to concentration maximum, time to maximum concentration, mean concentration over time (MC), area under the curve, odds, and time to return of spontaneous circulation (ROSC) in a normovolemic pediatric cardiac arrest model.

METHODS

Male swine weighing 24-37 kg were assigned to 4 groups: ET (n = 8), IV (n = 7), cardiopulmonary resuscitation (CPR) + defibrillation (CPR + Defib) (n = 5), and CPR only (n = 3). Swine were placed arrest for 2 minutes, and then CPR was initiated for 2 minutes. Epinephrine (0.1 mg/kg) for the ET group or 0.01 mg/kg for the IV was administered every 4 minutes or until ROSC. Defibrillation started at 3 minutes and continued every 2 minutes for 30 minutes or until ROSC for all groups except the CPR-only group. Blood samples were collected over a period of 5 minutes.

RESULTS

The MC of plasma epinephrine for the IV group was significantly higher at the 30- and 60-second time points (P = 0.001). The ET group had a significantly higher MC of epinephrine at the 180- and 240-second time points (P < 0.05). The concentration maximum of plasma epinephrine was significantly lower for the ET group (195 ± 32 ng/mL) than for the IV group (428 ± 38 ng/mL) (P = 0.01). The time to maximum concentration was significantly longer for the ET group (145 ± 26 seconds) than for the IV group (42 ± 16 seconds) (P = 0.01). No significant difference existed in area under the curve between the 2 groups (P = 0.62). The odds of ROSC were 7.7 times greater for the ET versus IV group. Time to ROSC was not significantly different among the IV, ET, and CPR + Defib groups (P = 0.31).

CONCLUSIONS

Based on the results of this study, the ET route of administration should be considered a first-line intervention.

Tibial Intraosseous Administration of Epinephrine Is Effective in Restoring Return of Spontaneous Circulation in a Pediatric Normovolemic But Not Hypovolemic Cardiac Arrest Model

OBJECTIVE

We compared the efficacy of tibial intraosseous (TIO) administration of epinephrine in a pediatric normovolemic versus hypovolemic cardiac arrest model to determine the incidence of return of spontaneous circulation (ROSC) and plasma epinephrine concentrations over time.

METHODS

This experimental study evaluated the pharmacokinetics of epinephrine and/or incidence of ROSC after TIO administration in either a normovolemic or hypovolemic pediatric swine model.

RESULTS

All subjects in the TIO normovolemia cardiac arrest group experienced ROSC after TIO administration of epinephrine. In contrast, subjects experiencing hypovolemia and cardiac arrest were significantly less likely to experience ROSC when epinephrine was administered TIO versus intravenous (TIO hypovolemia: 14% [1/7] vs IV hypovolemia: 71% [5/7]; P = 0.031). The TIO hypovolemia group exhibited significantly lower plasma epinephrine concentrations versus IV hypovolemia at 60, 90, 120, and 150 seconds (P < 0.05). Although the maximum concentration of plasma epinephrine was similar, the TIO hypovolemia group exhibited significantly slower time to maximum concentration times versus TIO normovolemia subjects (P = 0.004).

CONCLUSIONS

Tibial intraosseous administration of epinephrine reliably facilitated ROSC among normovolemic cardiac arrest pediatric patients, which is consistent with published reports. However, TIO administration of epinephrine was ineffective in restoring ROSC among subjects experiencing hypovolemia and cardiac arrest. Tibial intraosseous-administered epinephrine during hypovolemia and cardiac arrest may have resulted in a potential sequestration of epinephrine in the tibia. Central or peripheral intravascular access attempts should not be abandoned after successful TIO placement in the resuscitation of patients experiencing concurrent hypovolemia and cardiac arrest.

Age-related changes in CB1 receptor expression and function and the behavioral effects of cannabinoid receptor ligands

Cannabinoid use has increased among aging individuals. However, little information on age-related differences in the behavioral effects of these agents is available. To explore potential differences in the behavioral effects of cannabinoids, we determined effects of Δ⁹-tetrahydrocannabinol (THC, 1-10 mg/kg) or rimonabant (0.3-3.2 mg/kg) on operant fixed-ratio responding (FR10) for food in young adult (6 months) and aged (29 months) rats. THC dose-dependently decreased responding for food. Rimonabant alone had little or no effect on responding up to 1.0 mg/kg, but disrupted responding following a 3.2 mg/kg dose. Rimonabant (1.0 mg/kg) partially antagonized response disruption by THC. These effects were similar in young adult and aged rats. However, aging has been reported to change the neurobiology of cannabinoid CB1 receptors. To confirm our rats exhibited such differences, we assessed CB1 receptor binding sites and function in six subcortical (caudate, nucleus accumbens CA1, and CA2/CA3), and three cortical regions (medial prefrontal, temporal, entorhinal) in young adult (6 months) or aged (26 months) male Lewis rats using quantitative autoradiography. CB1 receptor binding sites were reduced in cortical, but not subcortical brain regions of aged rats. CB1 receptor function, at the level of receptor-G protein interaction, was not different in any region studied. Results indicate that down-regulation of CB1 receptor binding sites observed in cortical regions of aged rats was not accompanied by a commensurate decrease in CB1 receptor-stimulated [³⁵S]GTPγS binding, suggesting a compensatory increase in receptor function in cortical areas. Together, our results provide additional evidence of age-related changes in central CB1 receptor populations. However, the functional compensation for decreased CB1 receptor binding may mitigate changes in behavioral effects of cannabinoids. With the rising use of cannabinoid-based therapeutics among aging populations, further evaluation of age-related changes in the cannabinoid system and the impact of these changes on effects of this class of drugs is warranted.

Humerus Intraosseous and Intravenous Administration of Epinephrine in Normovolemic and Hypovolemic Cardiac Arrest Porcine Models

OBJECTIVE

The aim of this study was to compare area under the curve (AUC), frequency, and odds of return of spontaneous circulation (ROSC) when epinephrine was administered in hypovolemic and normovolemic cardiac arrest models.

METHODS

Twenty-eight adult swine were randomly assigned to 4 groups: HIO Normovolemia Group (HIONG); HIO Hypovolemia Group (HIOHG); IV Normovolemia (IVNG); and IV Hypovolemia Group (IVHG). Swine were anesthetized. The HIOH and IVH subjects were exsanguinated 35% of their blood volume. Each was placed into arrest. After 2 minutes, cardiopulmonary resuscitation was initiated. After another 2 minutes, 1 mg of epinephrine was given by IV or HIO routes; blood samples were collected over 5 minutes and analyzed by high-performance liquid chromatography. Subjects were defibrillated every 2 minutes.

RESULTS

The AUC in the HIOHG was significantly less than both the HIONG (p = 0.047) and IVHG (p = 0.021). There were no other significant differences in the groups relative to AUC (p > 0.05). HIONG had a significantly higher occurrence of ROSC compared to HIOHG (p = 0.018) and IVH (p =0.018) but no other significant differences (p > 0.05). The odds of ROSC were 19.2 times greater for HIONG compared to the HIOHG.

CONCLUSION

The study strongly supports the effectiveness of HIO administration of epinephrine and should be considered as a first-line intervention for patients in cardiac arrest related to normovolemic causes. However, our findings do not support using HIO access for epinephrine administration for patients in cardiac arrest related to hypovolemic reasons.

Endotracheal Administered Epinephrine Is Effective in Return of Spontaneous Circulation Within a Pediatric Swine Hypovolemic Cardiac Arrest Model

OBJECTIVE

Early administration of epinephrine increases the incidence of return of spontaneous circulation (ROSC) and improves outcomes among pediatric cardiac arrest victims. Rapid endotracheal (ET) intubation can facilitate early administration of epinephrine to pediatric victims. To date, no studies have evaluated the use of ET epinephrine in a pediatric hypovolemic cardiac arrest model to determine the incidence of ROSC.

METHODS

This prospective, experimental study evaluated the pharmacokinetics and/or incidence of ROSC following ET administered epinephrine and compared it to these experimental groups: intravenous (IV) administered epinephrine, cardiopulmonary resuscitation only (CPR), and CPR + defibrillation (CPR + Defib).

RESULTS

Endotracheal administered epinephrine, at the Pediatric Advanced Life Support (PALS) recommended dose, was not significantly different than IV administered epinephrine in maximum plasma concentrations, time to maximum plasma concentration, area under the curve, or ROSC, or mean plasma concentrations at various time points (P > 0.05). The odds of ROSC in the ET group were 2.4 times greater than the IV group. The onset to ROSC in the ET group was significantly shorter than the IV group (P < 0.0001).

CONCLUSIONS

These data support that ET epinephrine administration remains an alternative to IV administered epinephrine and faster at restoring ROSC among pediatric hypovolemic cardiac arrest victims in the acute setting when an endotracheal tube is present. Although further research is required to determine long-term outcomes of high-dose ET epinephrine administration, these data reinforce the therapeutic potential of ET administration of epinephrine to restore ROSC before IV access.

The long-term effect of maternal dietary protein restriction on 5-HT1A receptor function and behavioral responses to stress in adulthood

Maternal nutrition impacts fetal development, and may play a role in determining resilience to stress and vulnerability to stress-precipitated psychiatric disorders, such as anxiety and depression. In this study, we examined the effect of a reduction in maternal dietary protein during pregnancy on the brain neurochemistry and behavior of offspring. We focused specifically on the serotonin system, the 5-HT_1A receptor and the responsivity of offspring as adults to stress. Dams were fed either a low protein diet (10% protein by weight) or isocaloric control diet (20% protein by weight). The low protein diet did not alter maternal food intake and body weight, or litter size and the average birth weight of male or female littermates. 5-HT_1A receptor function, as measured by quantitative autoradiography of 8-OH-DPAT (1 μM)-stimulated [³⁵S]GTPγS binding, was markedly reduced in hippocampus of weanling female, but not male offspring (postnatal day, PND 21) of dams fed the low protein diet. The number of serotonergic cell bodies in the rostral raphe, and 5-HT metabolism in the limbic system of weanling offspring was not altered by maternal low protein diet. The deficit in hippocampal 5-HT_1A receptor function observed in weanling female offspring persisted into adulthood (PND 112), and was accompanied by an increased sensitivity to stress, specifically increased immobility during a 15-minute forced swim challenge and increased anorexia following 30-minute restraint (PND 97-100). The present work begins to uncover important future directions for understanding the early developmental origins of resilience to stress, and factors that may put individuals at greater risk for stress-related psychiatric disorders.

Effects of selective serotonin and norepinephrine reuptake inhibitors on depressive- and impulsive-like behaviors and on monoamine transmission in experimental temporal lobe epilepsy

OBJECTIVE

Examine therapeutic potential of a selective serotonin reuptake inhibitor (SSRI) and a norepinephrine reuptake inhibitor (NERI) in an animal model of comorbidity between epilepsy, depression-like, and impulsive-like impairments.

METHODS

Epilepsy was induced in male Wistar rats by LiCl and pilocarpine. An SSRI fluoxetine (FLX), and an NERI reboxetine (RBX) were administered either alone or as a combination over 1 week. Depressive-like and impulsive-like behaviors were examined using the forced swim test. Fast scan cyclic voltammetry was used to analyze serotonergic transmission in the raphe nucleus (RN)-prefrontal cortex (PFC) pathway, and noradrenergic transmission in locus coeruleus (LC)-PFC, and LC-RN projections. Monoamine levels in PFC were measured using high-performance liquid chromatography (HPLC). Functional capacities of 5-HT1A receptors and α2A adrenoreceptors in PFC were analyzed by autoradiography.

RESULTS

Epileptic rats showed behavioral signs of depression and hyperimpulsivity, suppressed serotonergic and noradrenergic tones, decreased levels of serotonin (5-HT), and norepinephrine (NE); 5-HT1A receptor and α2A adrenoreceptors functions remained intact. FLX failed to improve behavioral deficits, but effectively raised 5-HT level and marginally improved RN-PFC serotonergic transmission. RBX reversed impulsive-like behavior, normalized content of NE and noradrenergic tone in LC-PFC and LC-RN. FLX-RBX combination fully reversed depressive-like behavior, and normalized RN-PFC serotonergic transmission. None of the treatment modified the function of 5-HT and NE receptors.

SIGNIFICANCE

Depressive- and impulsive-like behaviors in the pilocarpine model of epilepsy stem respectively from dysfunctions of serotonergic and noradrenergic ascending pathways. At the same time, epilepsy-associated depression is SSRI resistant. The finding that an SSRI-NERI combination exerts antidepressant effect, along with RBX-induced improvement of LC-RN noradrenergic transmission point toward the involvement of LC-RN noradrenergic input in enabling therapeutic potential of FLX. Medications that improve serotonergic and noradrenergic transmission, such as serotonin-norepinephrine reuptake inhibitors may be effective in treating epilepsy-associated SSRI-resistant depression, as well as concurrent depression and attention-deficit/hyperactivity disorder (ADHD).

Low-Level Stress Induces Production of Neuroprotective Factors in Wild-Type but Not BDNF+/- Mice: Interleukin-10 and Kynurenic Acid

BACKGROUND

Brain-derived neurotrophic factor (BDNF) deficiency confers vulnerability to stress, but the mechanisms are unclear. BDNF(+/-) mice exhibit behavioral, physiological, and neurochemical changes following low-level stress that are hallmarks of major depression. After immune challenge, neuroinflammation-induced changes in tryptophan metabolism along the kynurenine pathway mediate depressive-like behaviors.

METHODS

We hypothesized that BDNF(+/-) mice would be more susceptible to stress-induced neuroinflammation and kynurenine metabolism, so BDNF(+/-) or wild-type littermate mice were subject to repeated unpredictable mild stress. Proinflammatory cytokine expression and kynurenine metabolites were measured.

RESULTS

Unpredictable mild stress did not induce neuroinflammation. However, only wild-type mice produced the neuroprotective factors interleukin-10 and kynurenic acid in response to repeated unpredictable mild stress. In BDNF(+/-) mice, kynurenine was metabolized preferentially to the neurotoxic intermediate 3-hydroxykynurenine following repeated unpredictable mild stress.

CONCLUSIONS

Our data suggest that BDNF may modulate kynurenine pathway metabolism during stress and provide a novel molecular mechanism of vulnerability and resilience to the development of stress-precipitated psychiatric disorders.

Enhanced novelty-induced corticosterone spike and upregulated serotonin 5-HT1A and cannabinoid CB1 receptors in adolescent BTBR mice

Hypothalamic pituitary adrenal (HPA) axis responses to change and social challenges during adolescence can influence mental health and behavior into adulthood. To examine how HPA tone in adolescence may contribute to psychopathology, we challenged male adolescent (5 weeks) and adult (16 weeks) BTBR T(+)tf/J (BTBR) and 129S1/SvImJ (129S) mice with novelty in sociability tests. In prior studies these strains had exaggerated or altered HPA stress responses and low sociability relative to C57BL/6J mice in adulthood. In adolescence these strains already exhibited similar or worse sociability deficits than adults or age-matched C57 mice. Yet BTBR adolescents were less hyperactive and buried fewer marbles than adults. Novelty-induced corticosterone (CORT) spikes in adolescent BTBR were double adult levels, and higher than 129S or C57 mice at either age. Due to their established role in HPA feedback, we hypothesized that hippocampal Gαi/o-coupled serotonin 5-HT1A and cannabinoid CB1 receptor function might be upregulated in BTBR mice. Adolescent BTBR mice had higher hippocampal 5-HT1A density as measured by [(3)H] 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) binding than C57 mice, and adult BTBR 8-OH-DPAT-stimulated GTPγS binding was higher than in either C57 or 129S mice in this region. Further, BTBR hippocampal CB1 density measured by [(3)H]CP55,940 binding was 15-20% higher than in C57. CP55,940-stimulated GTPγS binding in adult BTBR dentate gyrus was 30% higher then 129S (p<0.05), but was not a product of greater neuronal or cell density defined by NeuN and DAPI staining. Hence hyperactive HPA responsiveness during adolescence may underlie 5-HT1A and CB1 receptor up-regulation and behavioral phenotype of BTBR mice.

Reduced BDNF attenuates inflammation and angiogenesis to improve survival and cardiac function following myocardial infarction in mice

Brain-derived neurotrophic factor (BDNF) increases in failing hearts, but BDNF roles in cardiac remodeling following myocardial infarction (MI) are unclear. Male BDNF(+/+) [wild-type (WT)] and BDNF(+/-) heterozygous (HET) mice at 6-9 mo of age were subjected to MI and evaluated at days 1, 3, 5, 7, or 28 post-MI. At day 28 post-MI, 76% of HET versus 40% of WT survived, whereas fractional shortening improved and neovascularization levels were reduced in the HET (all, P < 0.05). At day 1, post-MI, matrix metalloproteinase-9, and myeloperoxidase (MPO) increased in WT, but not in HET. Concomitantly, monocyte chemotactic protein-1 and -5 levels increased and vascular endothelial growth factor (VEGF)-A decreased in HET. Neutrophil infiltration peaked at days 1-3 in WT mice, and this increase was blunted in HET. To determine if MPO administration could rescue the HET phenotype, MPO was injected at 3 h post-MI. MPO restored VEGF-A levels without altering matrix metalloproteinase-9 or neutrophil content. In conclusion, reduced BDNF levels modulated the early inflammatory and neovascularization responses, leading to improved survival and reduced cardiac remodeling at day 28 post-MI. Thus reduced BDNF attenuates early inflammation following MI by modulating MPO and angiogenic response through VEGF-A.

Hippocampal serotonin-1A receptor function in a mouse model of anxiety induced by long-term voluntary wheel running

We have recently demonstrated that, in C57/Bl6 mice, long-term voluntary wheel running is anxiogenic, and focal hippocampal irradiation prevents the increase in anxiety-like behaviors and neurobiological changes in the hippocampus induced by wheel running. Evidence supports a role of hippocampal 5-HT1A receptors in anxiety. Therefore, we investigated hippocampal binding and function of 5-HT1A receptors in this mouse model of anxiety. Four weeks of voluntary wheel running resulted in hippocampal subregion-specific changes in 5-HT1A receptor binding sites and function, as measured by autoradiography of [(3) H] 8-hydroxy-2-(di-n-propylamino)tetralin binding and agonist-stimulated binding of [(35) S]GTPγS to G proteins, respectively. In the dorsal CA1 region, 5-HT1A receptor binding and function were not altered by wheel running or irradiation. In the dorsal dentate gyrus and CA2/3 region, 5-HT1A receptor function was decreased by not only running but also irradiation. In the ventral pyramidal layer, wheel running resulted in a decrease of 5-HT1A receptor function, which was prevented by irradiation. Neither irradiation nor wheel running affected 5-HT1A receptors in medial prefrontal cortex or in the dorsal or median raphe nuclei. Our data indicate that downregulation of 5-HT1A receptor function in ventral pyramidal layer may play a role in anxiety-like behavior induced by wheel running.

Catecholamine/Serotonin interactions: systems thinking for brain function and disease

This chapter brings together the work of several leading laboratories, each an outstanding example of integrative approaches to complex diseases of the central nervous system. Cognitive dysfunction and negative symptoms associated with schizophrenia are believed to result from hypofunction of the mesocortical dopaminergic projections to prefrontal cortex (PFC). Noradrenergic targets for the augmentation of dopaminergic function in PFC show promise to improve cognitive deficits as well as negative symptoms. Serotonergic targets for the modulation of mesocortical dopaminergic neurotransmission include 5-HT2A and 5-HT1A receptors. The hallmark of Parkinson's disease is the destruction of nigrostriatal dopaminergic neurons. l-DOPA, a metabolic precursor of dopamine, is the standard of treatment. However, the ectopic release of dopamine (DA) from serotonin neurons and the clearance of extracellular DA by the norepinephrine transporter in areas enriched with noradrenergic terminals contribute to extracellular DA produced by l-DOPA and offer opportunities to improve l-DOPA therapy. The high-affinity transporters for monoamines are the primary targets for antidepressant drugs. However, many patients experience suboptimal therapeutic benefit or fail to respond to treatment. Organic cation transporters and plasma membrane monoamine transporter serve an important function in regulating monoamine neurotransmission and hold potential utility as targets for the development of therapeutic drugs. Improved therapeutic approaches will arise from not only understanding how monoamines influence one another within the central nervous system as an integrated whole but also addressing the pathophysiology of specific core symptoms or distinct syndromal dimensions (cognitive impairment, motor slowing, and negative affect) regardless of disease classification, for example, psychotic, affective, and neurodegenerative.

Interleukin-1β causes fluoxetine resistance in an animal model of epilepsy-associated depression

Depression represents a common comorbidity of epilepsy and is frequently resistant to selective serotonin reuptake inhibitors (SSRI). We tested the hypothesis that the SSRI resistance in epilepsy associated depression may be a result of a pathologically enhanced interleukin-1β (IL1-β) signaling, and consequently that the blockade of IL1-β may restore the effectiveness of SSRI. Epilepsy and concurrent depression-like impairments were induced in Wistar rats by pilocarpine status epilepticus (SE). The effects of the 2-week long treatment with fluoxetine, interleukin-1 receptor antagonist (IL-1ra), and their combination were examined using behavioral, biochemical, neuroendocrine, and autoradiographic assays. In post-SE rats, depression-like impairments included behavioral deficits indicative of hopelessness and anhedonia; the hyperactivity of the hypothalamo-pituitary-adrenocortical axis; the diminished serotonin output from raphe nucleus; and the upregulation of presynaptic serotonin 1-A (5-HT1A) receptors. Fluoxetine monotherapy exerted no antidepressant effects, whereas the treatment with IL-1ra led to the complete reversal of anhedonia and to a partial improvement of all other depressive impairments. Combined administration of fluoxetine and IL-1ra completely abolished all hallmarks of epilepsy-associated depressive abnormalities, with the exception of the hyperactivity of the hypothalamo-pituitary-adrenocortical axis, the latter remaining only partially improved. We propose that in certain forms of depression, including but not limited to depression associated with epilepsy, the resistance to SSRI may be driven by the pathologically enhanced interleukin-1β signaling and by the subsequent upregulation of presynaptic 5-HT1A receptors. In such forms of depression, the use of interleukin-1β blockers in conjunction with SSRI may represent an effective therapeutic approach.

GABAB receptor-positive modulators: brain region-dependent effects

This study examined the positive modulatory properties of 2,6-di-tert-butyl-4-(3-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930) and (R,S)-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2-one (rac-BHFF) at γ-aminobutyric acid B (GABA(B)) receptors in different brain regions. Using quantitative autoradiography, we measured GABA(B) receptor-stimulated binding of guanosine 5'-O-(3-[³⁵S]thiotriphosphate) ([³⁵S]GTPγS) to G proteins in medial prefrontal cortex (mPFC), hippocampus, and cerebellum. CGP7930 and rac-BHFF enhanced baclofen-stimulated [³⁵S]GTPγS binding similarly in mPFC and hippocampus, but were more effective in cerebellum. CGP7930 (100 μM) increased [³⁵S]GTPγS binding stimulated by baclofen (30 μM) from 29 to 241% above basal in mPFC and from 13 to 1530% above basal in cerebellum. Likewise, rac-BHFF (10 μM) increased baclofen-stimulated [³⁵S]GTPγS binding more in cerebellum (from 13 to 1778% above basal) than in mPFC (from 29 to 514% above basal). rac-BHFF (10 μM) in combination with γ-hydroxybutyrate (20 mM) increased [³⁵S]GTPγS binding in cerebellum but not in mPFC. rac-BHFF also enhanced the effects of 3-aminopropyl(diethoxymethyl)phosphinic acid (CGP35348). Consistent with its partial agonist properties, CGP35348 stimulated [³⁵S]GTPγS binding in mPFC when given alone (to 18% above basal), but less extensively than baclofen (140% above basal), and antagonized baclofen when given together. CGP35348 (1 mM) in combination with rac-BHFF (100 μM) produced an increase in [³⁵S]GTPγS binding that was larger in cerebellum (from 61 to 1260% above basal) than in mPFC (from 18 to 118% above basal). Taken together, the results show that GABA(B) receptor-positive modulators enhance [³⁵S]GTPγS binding stimulated by GABA(B) receptor agonists in a brain region-dependent manner. This regionally selective enhancement is further evidence of pharmacologically distinct GABA(B) receptor populations, possibly allowing for more selective therapeutic targeting of the GABA(B) system.

Plasticity of presynaptic and postsynaptic serotonin 1A receptors in an animal model of epilepsy-associated depression

Depression is a common comorbidity of temporal lobe epilepsy and has highly negative impact on patients' quality of life. We previously established that pilocarpine-induced status epilepticus (SE) in rats, concurrently with chronic epilepsy leads to depressive impairments, and that the latter may stem from the dysregulation of hypothalamo-pituitary-adrenocortical (HPA) axis and/or diminished raphe-hippocampal serotonergic transmission. We examined possible involvement of presynaptic and postsynaptic serotonin 1A (5-HT1A) receptors in epilepsy-associated depression. Based on their performance in the forced swim test (FST), post-SE animals were classified as those with moderate and severe depressive impairments. In moderately impaired rats, the activity of the HPA axis (examined using plasma corticosterone radioimmunoassay) was higher than in naive subjects, but the functional capacity of presynaptic 5-HT1A receptors (measured in raphe using autoradiography) remained unaltered. In severely depressed animals, both the activity of the HPA axis and the function of presynaptic 5-HT1A receptors were increased as compared with naive and moderately depressed rats. Pharmacological uncoupling of the HPA axis from raphe nucleus exerted antidepressant effects in severely impaired rats, but did not modify behavior in both naive and moderately depressed animals. Further, the function of postsynaptic 5-HT1A receptors was diminished in the hippocampus of post-SE rats. Pharmacological activation of postsynaptic 5-HT1A receptors improved depressive deficits in epileptic animals. We suggest that under the conditions of chronic epilepsy, excessively hyperactive HPA axis activates presynaptic 5-HT1A receptors, thus shifting the regulation of serotonin release in favor of autoinhibition. Downregulation of postsynaptic 5-HT1A receptors may further exacerbate the severity of epilepsy-associated depression.

Density and function of central serotonin (5-HT) transporters, 5-HT1A and 5-HT2A receptors, and effects of their targeting on BTBR T+tf/J mouse social behavior

BTBR mice are potentially useful tools for autism research because their behavior parallels core social interaction impairments and restricted-repetitive behaviors. Altered regulation of central serotonin (5-HT) neurotransmission may underlie such behavioral deficits. To test this, we compared 5-HT transporter (SERT), 5-HT(1A) and 5-HT(2A) receptor densities among BTBR and C57 strains. Autoradiographic [(3) H] cyanoimipramine (1 nM) binding to SERT was 20-30% lower throughout the adult BTBR brain as compared to C57BL/10J mice. In hippocampal membrane homogenates, [(3) H] citalopram maximal binding (B(max) ) to SERT was 95 ± 13 fmol/mg protein in BTBR and 171 ± 20 fmol/mg protein in C57BL/6J mice, and the BTBR dissociation constant (K(D) ) was 2.0 ± 0.3 nM versus 1.1 ± 0.2 in C57BL/6J mice. Hippocampal 5-HT(1A) and 5-HT(2A) receptor binding was similar among strains. However, 8-OH-DPAT-stimulated [(35) S] GTPγS binding in the BTBR hippocampal CA(1) region was 28% higher, indicating elevated 5-HT(1A) capacity to activate G-proteins. In BTBR mice, the SERT blocker, fluoxetine (10 mg/kg) and the 5-HT(1A) receptor partial-agonist, buspirone (2 mg/kg) enhanced social interactions. The D(2) /5-HT(2) receptor antagonist, risperidone (0.1 mg/kg) reduced marble burying, but failed to improve sociability. Overall, altered SERT and/or 5-HT(1A) functionality in hippocampus could contribute to the relatively low sociability of BTBR mice.

Regulation of cortical and hippocampal 5-HT(1A) receptor function by corticosterone in GR+/- mice

Our objective in the present study was to examine 5-HT(1A) receptor function in prefrontal cortex and hippocampus of GR+/- mice, which appear to be an appropriate murine model of depression. 5-HT(1A) receptor function was determined by measuring [(35)S]GTPgammaS binding stimulated by the 5-HT(1A) receptor agonist 8-OH-DPAT (1 microM), an indication of the capacity of the receptor to activate G proteins. 5-HT(1A) receptor expression was determined by measuring the binding of [(3)H]8-OH-DPAT (2 nM). We observed no effect of the constitutive reduction in GR on 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding or 5-HT(1A) receptor binding sites. Corticosterone treatment (10mg/kg, sc once daily for 21 days) of wild-type mice resulted in a decrease in 5-HT(1A) receptor function in prefrontal cortex [8-OH-DPAT-stimulated [(35)S]GTPgammaS binding (% above basal), vehicle-treated: 39+/-4.9; corticosterone-treated: 17+/-2.8], but not in hippocampus. The constitutive reduction in GR expression prevented the down-regulation of 5-HT(1A) receptor function in frontal cortex by chronic corticosterone administration. In contrast, corticosterone treatment of GR+/- mice resulted in an increase in 5-HT(1A) receptor function in hippocampus which reached statistical significance in CA2/3 region [8-OH-DPAT-stimulated [(35)S]GTPgammaS binding (% above basal), vehicle-treated: 41+/-9.7; corticosterone-treated: 94+/-23]. These changes seem to be evoked by a combined effect of high corticosterone levels and GR deficiency. Although GR+/- mice do not exhibit changes in baseline corticosterone, the constitutive deficiency in GR appears to have unmasked regulatory effects of elevated corticosterone in the maintenance of 5-HT(1A) receptor function in prefrontal cortex and hippocampus.

Chronic intermittent cold stress and serotonin depletion induce deficits of reversal learning in an attentional set-shifting test in rats

RATIONALE

Chronic stress perturbs modulatory brain neurotransmitter systems, including serotonin (5-HT), and is a risk factor for psychiatric disorders such as depression. Deficits in cognitive flexibility, reflecting prefrontal cortical dysfunction, are prominent in such disorders. Orbitofrontal cortex (OFC) has been implicated specifically in reversal learning, a form of cognitive flexibility modulated by 5-HT.

OBJECTIVES

The objectives of the study were (1) to assess the effects of chronic intermittent cold (CIC) stress, a potent metabolic stressor, on performance of rats in an attentional set-shifting test (AST), and (2) to assess a possible role for serotonin in CIC-induced deficits and test the effects of acute serotonin reuptake blockade.

MATERIALS AND METHODS

Male Sprague-Dawley rats were exposed to CIC stress (14 days x 6 h/day at 4 degrees C) before testing on the AST. In subsequent experiments, brain 5-HT was depleted in naïve rats with para-chlorophenylalanine or 5-HT release was increased acutely in CIC-stressed rats with citalopram (5 mg/kg, s.c.) given 30 min prior to the first reversal task. Microdialysis was used to assess CIC-induced changes in 5-HT release in OFC during testing.

RESULTS

CIC-stressed rats exhibited a selective impairment on the first reversal task in the AST. 5-HT depletion induced a similarly selective deficit in reversal learning. The CIC-induced impairment in reversal learning was attenuated by acute 5-HT reuptake blockade. 5-HT release was reduced in OFC of CIC-stressed rats during behavioral testing.

CONCLUSIONS

The CIC stress-induced impairment of cognitive flexibility may involve dysregulation of 5-HT modulatory function in OFC. Such deficits may thus model relevant symptoms of neuropsychiatric disorders that respond positively to SSRI treatment.

Differential regulation of serotonin-1A receptor-stimulated [35S]GTP gamma S binding in the dorsal raphe nucleus by citalopram and escitalopram

The effect of chronic citalopram or escitalopram administration on 5-HT1A receptor function in the dorsal raphe nucleus was determined by measuring [35S]GTP gamma S binding stimulated by the 5-HT1A receptor agonist (R)-(+)-8-OH-DPAT (1nM-10 microM). Although chronic administration of citalopram or escitalopram has been shown to desensitize somatodendritic 5-HT1A autoreceptors, we found that escitalopram treatment decreased the efficacy of 5-HT1A receptors to activate G proteins, whereas citalopram treatment did not. The binding of [3H]8-OH-DPAT to the coupled, high affinity agonist state of the receptor was not altered by either treatment. Interestingly, escitalopram administration resulted in greater occupancy of serotonin transporter sites as measured by the inhibition of [3H]cyanoimipramine binding. As the binding and action of escitalopram is limited by the inactive enantiomer R-citalopram present in racemic citalopram, we propose that the regulation of 5-HT1A receptor function in the dorsal raphe nucleus at the level of receptor-G protein interaction may be a result of greater inhibition of the serotonin transporter by escitalopram.

Serotonin-1A receptor function in the dorsal raphe nucleus following chronic administration of the selective serotonin reuptake inhibitor sertraline

Serotonin-1A (5-HT(1A) receptors in the dorsal raphe nucleus (DRN) function as somatodendritic autoreceptors, and therefore play a critical role in controlling serotonergic cell firing and serotonergic neurotransmission. We hypothesized that a decrease in the capacity of 5-HT(1A) receptors to activate G proteins was a general mechanism by which 5-HT(1A) receptors in the DRN are desensitized following chronic administration of selective serotonin reuptake inhibitors (SSRIs). Using in vivo microdialysis, we found that the ability of the 5-HT(1A) receptor agonist 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT) (0.025 mg/kg, s.c.) to decrease extracellular 5-HT levels in striatum was attenuated following chronic treatment of rats with the SSRIs sertraline or fluoxetine. This apparent desensitization of somatodendritic 5-HT(1A) autoreceptor function was not accompanied by a decrease in 5-HT(1A) receptor sites in the coupled, high-affinity agonist state as measured by the binding of [3H]8-OH-DPAT. In marked contrast to what was observed following chronic administration of fluoxetine, 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in the DRN was not altered following chronic sertraline treatment. Thus, desensitization of 5-HT(1A) somatodendritic autoreceptor function following chronic sertraline administration appears not to be due to a decrease in the capacity 5-HT(1A) receptors to activate G proteins in the DRN. Our findings suggest that the SSRIs may not be a homogeneous class of antidepressant drug with regard to the mechanism by which the function of somatodendritic 5-HT(1A) autoreceptors is regulated.

Effect of early rearing conditions on alcohol drinking and 5-HT1A receptor function in C57BL/6J mice

We have evaluated in C57BL/6J mice the effect of maternal separation and post-weaning social isolation on ethanol intake, and on serotonin1A (5-HT1A) receptor function at the level of receptor-G protein interaction in the hippocampus and dorsal raphe nucleus. From postnatal days 2-14, litters were separated from the mother for 15 min (Handled) or for 180 min (Maternal separation). After weaning, pups were housed in pairs or in social isolation. At 2 months of age, ethanol intake and preference in mice were assessed using the two-bottle choice paradigm. Maternal separation increased ethanol preference in female mice that were subsequently housed in isolation. By contrast, post-weaning isolation increased ethanol preference and consumption in male mice regardless of pre-weaning rearing conditions. The increased ethanol preference and intake were limited to a 5% (v/v) concentration of ethanol. Our data suggest that adolescent mice are susceptible to the effects of post-weaning social isolation as shown by increased ethanol preference and consumption. Using quantitative autoradiography, 5-HT1A receptor number and function were determined by the binding of [3H]WAY-100635, and by [35S]GTPgammaS binding stimulated by the 5-HT1A receptor agonist 8-OH-DPAT, respectively. The binding experiments were done at approximately 3 months after the end of the two-bottle choice test in an attempt to minimize direct effects of ethanol drinking on 5-HT1A receptor function and number. 5-HT1A receptor-stimulated [35S]GTPgammaS binding in the dorsal raphe nucleus was increased in animals reared after weaning in isolation vs. in pairs, regardless of gender or pre-weaning rearing conditions. Our data suggest that there are long-term neurochemical consequences of social isolation of adolescent mice, specifically increased 5-HT1A receptor function in the dorsal raphe nucleus.

Regulation of serotonin-1A receptor function in inducible brain-derived neurotrophic factor knockout mice after administration of corticosterone

BACKGROUND

We examined the effects of a forebrain-specific reduction in brain-derived neurotrophic factor (BDNF) on the regulation of serotonin-1A (5-HT1A) receptor function in serotonergic cell body areas as well as in limbic and cortical structures of mice chronically treated with corticosterone.

METHODS

5-HT1A receptor function, at the level of receptor-G protein interaction, was assessed with quantitative autoradiography of [35S]GTPgammaS binding stimulated by the 5-HT1A receptor agonist 8-OH-DPAT. 5-HT1A receptor number was assessed by measuring the binding of the antagonist radioligand [3H] WAY100635.

RESULTS

We observed a significant attenuation of 5-HT1A receptor function, in the absence of a change in receptor number, in the dorsal hippocampus of BDNF knockout versus control mice. There was no difference between control and BDNF knockout mice in 5-HT1A receptor number or function in the dorsal or median raphe nuclei or medial prefrontal cortex or anterior cingulate cortex. Corticosterone treatment of control mice decreased 5-HT1A receptor function in the dorsal and median raphe but not in hippocampus or frontal cortical areas. The regulation of 5HT1A receptor number or function in the dorsal and median raphe by corticosterone was lost in BDNF knockout mice.

CONCLUSIONS

Attenuation of BDNF expression in forebrain regions produces differential effects on distinct 5-HT1A receptor populations and on the regulation of these receptor populations by corticosterone.

Serotonin transporter function, but not expression, is dependent on brain-derived neurotrophic factor (BDNF): in vivo studies in BDNF-deficient mice

In the present study, we used high-speed chronoamperometry to examine serotonin (5-HT) transporter (5-HTT) function in vivo in 2-, 5-, and 10-month-old brain-derived neurotrophic factor (BDNF)+/- mice. The rate of clearance of exogenously applied 5-HT was measured in CA3 region of hippocampus. In 2-month-old mice, the rate of 5-HT clearance did not differ between BDNF+/+ and BDNF+/- mice. In BDNF+/+ mice, 5-HT clearance rate (Tc) increased markedly with age. In contrast, Tc remained relatively static in BDNF+/- mice across 2-, 5-, and 10-month age groups. At 5 months of age, female BDNF+/+ mice had a lower maximal velocity (Vmax) for 5-HT clearance than male BDNF+/+ mice. There was a similar trend in 5-month-old BDNF+/- mice, but this did not reach statistical significance. There was an age-dependent increase in KT value for 5-HT clearance (i.e., decreased in vivo affinity of 5-HTT), but no significant effect of genotype or gender. 5-HTT density, as measured by [3H]cyanoimipramine binding, was not different between BDNF+/+ and BDNF+/- mice, although there was a significant increase in 5-HTT binding with age. The selective 5-HT reuptake inhibitor fluvoxamine (50 and 100 pmol) significantly decreased 5-HT clearance in BDNF+/+ mice, but not in BDNF+/- mice. Our data suggest that the profoundly reduced ability of 5- and 10-month-old BDNF+/- mice to clear 5-HT is not because of a decrease in the total number of 5-HTTs, but may be due to functional deficits in the 5-HTT, e.g., in the machinery/signaling required for insertion of 5-HTTs into the plasma membrane and/or activation of the 5-HTT once it is positioned to take up 5-HT from extracellular fluid.

Chronic administration of venlafaxine fails to attenuate 5-HT1A receptor function at the level of receptor-G protein interaction

In this study venlafaxine was administered to rats at a low, moderate or high dose; for comparison, the selective serotonin reuptake inhibitor (SSRI) sertraline and the tricyclic antidepressant (TCA) amitriptyline were also included. We evaluated, using quantitative autoradiography, the effect of these antidepressant treatments on [35S]GTPgammaS binding stimulated by the 5-HT1A receptor agonist 8-OH-DPAT, a measure of the capacity of 5-HT1A receptors to activate G proteins. Chronic administration of amitriptyline resulted in a marked increase in 5-HT1A receptor-stimulated [35S]GTPgammaS binding in the hippocampus which was accompanied by an increase in 5-HT1A receptor number. 5-HT1A receptor-stimulated [35S]GTPgammaS binding in the hippocampus was also increased by chronic treatment with the highest dose of venlafaxine; 5-HT1A receptor number, however, was not significantly altered. In serotonergic cell body areas (i.e. dorsal and median raphe nuclei), 5-HT1A receptor-stimulated [35S]GTPgammaS binding was not altered by chronic administration of amitriptyline, sertraline or venlafaxine. Chronic TCA treatment does not desensitize somatodendritic 5-HT1A autoreceptor function. However, the lack of effect of chronic sertraline treatment on 5-HT1A receptor-stimulated [35S]GTPgammaS binding is in contrast to what has been observed previously following chronic administration of the SSRI fluoxetine, and suggests that different SSRIs may regulate somatodendritic 5-HT1A autoreceptor function differently depending on their pharmacology. Our data also suggest that the desensitization of somatodendritic 5-HT1A autoreceptors observed in electrophysiological studies following chronic venlafaxine administration is not at the level of receptor-G protein interaction. The hypothermic response in vivo to acute injection of 8-OH-DPAT was significantly attenuated following chronic treatment with venlafaxine or sertraline, but not amitriptyline.

Fluoxetine disrupts food intake and estrous cyclicity in Fischer female rats

Adult, regularly cycling female Fischer rats were injected daily with 10 mg/kg fluoxetine for 12-23 days. In the first experiment, body weight and vaginal smears were monitored daily. Fluoxetine treatment reduced body weight within the first 24 h of treatment. Fluoxetine treatment also elongated the estrous cycle, reduced blood levels of progesterone, and eliminated lordosis behavior. In the second experiment, body weight and food intake were examined and a pair-fed group was included to determine if fluoxetine-induced anorexia contributed to the disturbance of the estrous cycle. In pair-fed rats, effects similar to fluoxetine treatment were present. These results lead to the suggestion that fluoxetine's anorectic properties could disrupt the female's normal endocrine cyclicity and that this disruption could be relevant to the reduction in sexual behavior and motivation. However, when the duration of fluoxetine treatment was extended beyond 16 to 17 days, fluoxetine-treated female rats reinitiated vaginal cyclicity and showed evidence of normal sexual receptivity. In contrast, the estrous cycles of their pair-fed counterparts remained disrupted. Thus, restricted food intake appears to contribute to the disruption of the estrous cycle and elimination of sexual receptivity during fluoxetine treatment. However, compensatory changes in the serotonergic system that are associated with chronic fluoxetine administration may contribute to the gradual recovery of estrous cyclicity and sexual receptivity of the fluoxetine-treated animals.

Serotonergic modulation of the limbic system

The limbic system is composed of cortical as well as subcortical structures, which are intimately interconnected. The resulting macrostructure is responsible for the generation and expression of motivational and affective states. Especially high levels of serotonin are found in limbic forebrain structures. Serotonin projections to these structures, which arise from serotonergic cell body groups in the midbrain, form a dense plexus of axonal processes. In many areas of the limbic system, serotonergic neurotransmission can best be described as paracrine or volume transmission, and thus serotonin is believed to play a neuromodulatory role in the brain. Serotonergic projections to limbic structures, arising primarily from the dorsal and median raphe nuclei, compose two distinct serotonergic systems differing in their topographic organization, electrophysiological characteristics, morphology, as well as sensitivity to neurotoxins and perhaps psychoactive or therapeutic agents. These differences may be extremely important in understanding the role of these two serotonergic systems in normal brain function and in mental illness. Central serotonergic neurons or receptors are targets for a variety of therapeutic agents used in the treatment of disorders of the limbic system.

Autoreceptor-mediated inhibition of norepinephrine release in rat medial prefrontal cortex is maintained after chronic desipramine treatment

Alterations in noradrenergic neurotransmission are important in the mechanism of action of many antidepressant drugs, including selective norepinephrine (NA) reuptake inhibitors such as desipramine (DMI). It has been suggested that chronic NA reuptake blockade induces a desensitization of inhibitory alpha(2)-adrenergic autoreceptors. This hypothesis was tested in experiment 1 using in vivo microdialysis to examine the degree of alpha(2)-autoreceptor-mediated inhibition of NA release in rat medial prefrontal cortex exerted by endogenous NA following chronic treatment with vehicle or DMI. This was accomplished by measuring the elevation of extracellular NA levels induced by acute administration of the alpha(2)-receptor antagonist yohimbine. An 8-fold increase in basal NA levels was observed after 21 days of DMI treatment. Further, acute yohimbine administration induced a robust elevation in NA levels which was not attenuated, and in fact at lower doses was greater in DMI-treated rats compared with vehicle-treated controls. In experiment 2, we addressed directly the functional status of terminal alpha(2)-autoreceptors in frontal cortex in vitro, in the absence of potentially confounding competition from elevated levels of endogenous NA, after chronic reuptake blockade. We observed no difference in the degree to which the alpha(2)-receptor agonist clonidine inhibited potassium-evoked [(3)H]-NA release from cortical slices taken from DMI- or vehicle-treated rats. Together, these data suggest that endogenous activation of alpha(2)-autoreceptors persists in restraining NA neurotransmission in the face of tonically elevated basal NA levels following chronic reuptake blockade.

Effect of valproic acid on serotonin-2A receptor signaling in C6 glioma cells

Valproic acid (VPA), which has demonstrated efficacy in the treatment of bipolar disorder, has been shown to alter components of the phosphoinositide (PI) signaling cascade and to increase gene expression mediated by the transcription factor activator protein 1 (AP-1). Central serotonin-2A (5-HT2A) receptors, which have been implicated in the pathophysiology of manic-depressive illness, are coupled to PI hydrolysis. The promoter region of the 5-HT2A receptor gene contains AP-1 binding sites. We examined in C6 glioma cells the effect of VPA on 5-HT2A receptor signaling. Treatment of cells with VPA (100 microg/mL) for 20 h, but not 1.5 h, resulted in an enhancement of 5-HT2A receptor-stimulated PI hydrolysis. This effect of 20-h VPA exposure appeared not to be at the level of G protein or effector (i.e. phospholipase C: PLC) as inositol phosphate accumulation stimulated by aluminum fluoride or the PLC activator 2,4,6-trimethyl-N-(m-3-trifluromethylphenyl) benzenesulfonamide was not increased. The number of 5-HT2A receptors, as determined in saturation binding experiments using [3H]ketanserin, was increased by 20-h VPA treatment, with no change in affinity (KD). Taken together, our data suggest that the increase in 5-HT2A receptor-mediated PI hydrolysis following 20-h VPA exposure is not due to a general effect of VPA on this signaling cascade, but due to the up-regulation of 5-HT2A receptor number.

Reduced 5-HT3 receptor binding and lower baseline plus maze anxiety in the alcohol-preferring inbred fawn-hooded rat

The present investigation sought to explore the relationship between the 5-HT(3) receptor and anxiety-like behavior in fawn-hooded (FH/Wjd) rats, an inbred strain that exhibits a high intake and preference for ethanol, and the alcohol-nonpreferring ACI/N strain. Using quantitative autoradiography, we examined whether there were differences in central 5-HT(3) receptor binding in FH/Wjd versus ACI/N rats. Ten to 14 days prior to being used in the autoradiographic studies, rats were first confirmed to be representative of their strains by subjecting them to a two-bottle choice procedure for 2 weeks. The binding of [3H]LY 278584 to 5-HT(3) receptors was significantly reduced in frontal cortex, CA1 region of hippocampus, and in the medial and lateral nuclei of the amygdala of FH/Wjd versus ACI/N rats. In the anterior cingulate cortex and in the dentate gyrus region of the hippocampus the reduction in [3H]LY 278548 binding in the FH/Wjd versus ACI/N strain (40% and 41%, respectively) did not reach statistical significance. In a separate group of animals, the effects of the 5-HT(3) receptor antagonist MDL 72222 (3 mg/kg ip) on anxiety-related behaviors were assessed in the elevated plus maze. In vehicle-treated rats, the FH/Wjd strain exhibited significantly greater percent of time spent on the open arms and percent open arm entries, an indication of less anxiety. Pretreatment with MDL 72222 did not alter these behaviors in the FH/Wjd rats, but had an anxiolytic-like effect in the ACI/N strain, significantly increasing the percent of time spent on the open arms and percent open arm entries. Further research into 5-HT(3) receptor function in the alcohol-preferring FH/Wjd rats is needed to elucidate the relationship among 5-HT(3) receptors, alcohol drinking, and anxiety.

Ethanol consumption and serotonin-1A (5-HT1A) receptor function in heterozygous BDNF (+/-) mice

Heterozygous brain-derived neurotrophic factor (BDNF) (+/-) mice display abnormalities in central serotonergic neurotransmission, develop decrements in serotonergic innervation of the forebrain, and exhibit enhanced intermale aggressiveness. As disturbances of serotonin neurotransmission are implicated in alcohol abuse and aggression, we have examined in BDNF (+/-) mice alcohol drinking behavior, as well as central 5-hydroxytryptamine (5-HT)1A receptor function at the level of 5-HT1A receptor-G protein interaction. BDNF (+/-) mice displayed increased ethanol intake in a two-bottle choice procedure. There was no difference in the preference ratio for non-alcoholic tastants (i.e. quinine or saccharin) between genotypes. In the brains of alcohol-naive mice, we measured [35S]GTP gamma S binding stimulated by the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-dipropyl-aminotetralin hydrobromide (8-OH-DPAT; 1 microM). In BDNF (+/-) versus wild-type (WT) mice, 5-HT1A receptor-stimulated [35S]GTP gamma S binding was significantly attenuated in the median raphe nucleus. There was a decrease in (+/-)8-OH-DPAT-stimulated [35S]GTP gamma S binding in the dorsal raphe, which did not reach statistical significance. In the hippocampus, 5-HT1A receptor-stimulated [35S]GTP gamma S binding was significantly attenuated in BDNF (+/-) mice. 5-HT1A receptor-stimulated [35S]GTP gamma S binding was attenuated in the anterior cingulate cortex and lateral septum, although these reductions did not reach statistical significance. 5-HT1A receptor number was not different between genotypes in any area of brain examined, suggesting that 5-HT1A receptor function, specifically the capacity of the 5-HT1A receptor to activate G proteins, is attenuated in BDNF (+/-) mice.

Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration

Adaptive changes in the serotonergic system are generally believed to underlie the therapeutic effectiveness of the azapirone anxiolytics and a variety of antidepressant drugs. The serotonin-1A (5-HT(1A)) receptor has been implicated in affective disorders. Thus, studies of the regulation of 5-HT(1A) receptor function may have important implications for our understanding the role of this receptor in the mechanism of action of these therapeutic agents. This review focuses on the regulation of central 5-HT(1A) receptor function following administration of 5-HT(1A) receptor agonists or antidepressant drugs expected to increase the synaptic concentration of the neurotransmitter 5-HT. The majority of evidence supports regional differences in the regulation of central 5-HT(1A) receptor function following repeated agonist or antidepressant administration, which may be due to differences in processes involved in desensitization of the receptor at the cellular level. Region-specific differences in the regulation of 5-HT(1A) receptor function may be based on compensatory changes distal to the receptor, such as regulatory changes at the level of effector (e.g. adenylyl cyclase or ion channel), or at the level of the G protein such as changes in G protein expression, or phosphorylation of the G protein. It may be that the increase in serotonin neurotransmission, due to somatodendritic autoreceptor desensitization following agonist or antidepressant treatment, to normo-sensitive 5-HT(1A) receptors in certain brain regions (e.g. hippocampus or cortex) and to sub-sensitive 5-HT(1A) receptors in other brain regions (e.g. amygdala or hypothalamus) underlies the therapeutic efficacy of these drugs.

Selective heterologous regulation of 5-HT1A receptor-stimulated 35S GTPgammaS binding in the anterior cingulate cortex as a result of 5-HT2 receptor activation

Previous studies have shown that administration of the 5-HT(2) receptor agonist DOI to rats results in the heterologous desensitization of 5-HT(1A) receptor-mediated behavioral and neuroendocrine responses [Neuropsychopharmacology 19 (1998) 354; J. Neurosci. 21 (2001) 7919]. We hypothesized that the basis for these changes in 5-HT(1A) receptor function may involve changes in the capacity of the 5-HT(1A) receptor to activate G proteins. We examined the effect of chronic administration of DOI on the regulation of 5-HT(1A) receptor function at the level of receptor-G protein interaction using quantitative autoradiography of [(35)S]GTPgammaS binding stimulated by the 5-HT(1A) receptor agonist (+/-)8-OH-DPAT (1 microM). Repeated administration of DOI (1 mg/kg, s.c. once daily for 8 days) resulted in a marked down-regulation in 5-HT(2A) binding sites, as labeled by the antagonist radioligand [(3)H]ketanserin, throughout the cerebral cortex. Chronic DOI treatment also resulted in a significant and selective attenuation of 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in the anterior cingulate cortex (vehicle-treated: 74+/-7.7% above basal; DOI-treated: 43+/-4.6% above basal). Interestingly, 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding was not altered in the dorsal or median raphe, or in the limbic structures and other cortical regions examined. The decrease in 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in anterior cingulate cortex was not due to a decrease in 5-HT(1A) receptor number, indicating that the capacity of the 5-HT(1A) receptor to activate G proteins is attenuated in this cortical area following repeated DOI treatment. The heterologous regulation of 5-HT(1A) receptor function by chronic 5-HT(2) receptor activation in the anterior cingulate cortex raises interesting questions as to how the regulatory interaction between these serotonin receptor subtypes influences cognition, memory and emotion.

Aging alters in a region-specific manner serotonin transporter sites and 5-HT(1A) receptor-G protein interactions in hamster brain

Key proteins regulating serotonergic activity, specifically the serotonin transporter and 5-HT(1A) receptor, were examined in the midbrain raphe nuclei of young (3-4 months) and old (17-19 months) hamsters (N=7-10/group). An age-related decrease in the maximal density of serotonin transporter sites labelled with [(3)H]paroxetine (fmol/mg protein, Old: 396+/-13; Young: 487+/-27) was observed in the dorsal raphe nucleus (DRN) but not the median raphe nucleus (MRN), without affecting the affinity of [(3)H]paroxetine. In the DRN and MRN, the stimulation of [(35)S]GTP gamma S binding by the 5-HT(1A) receptor agonist 8-OH-DPAT, or the number of 5-HT(1A) receptor sites labeled with [(3)H] MPPF, was not different in old versus young animals. Thus in the DRN, aging decreased serotonin transporter sites without changing 5-HT(1A) receptor activation of G proteins or 5-HT(1A) receptor density. In the CA(1) region of hippocampus, 8-OH-DPAT-stimulated [(35)S]GTP gamma S binding was increased in the older animals (% above basal, Old: 141+/-21; Young: 81+/-17) without changing specific [(3)H] MPPF binding sites, suggesting that the capacity of 5-HT(1A) receptors to activate G proteins is enhanced. Aging also appears to enhance this capacity in the dentate gyrus, because this region exhibited a constant level of 8-OH-DPAT-stimulated [(35)S]GTP gamma S binding in spite of an age-related decrease in the number of [(3)H] MPPF binding sites (fmol/mg protein, Old: 203+/-21; Young: 429+/-51).

Differential regulation of 5-HT1A receptor-G protein interactions in brain following chronic antidepressant administration

Changes in 5-HT(1A) receptor function or sensitivity following chronic antidepressant treatment may involve changes in receptor-G protein interaction. We have examined the effect of chronic administration of the SSRI fluoxetine or the tricyclic antidepressant amitriptyline on 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in serotonergic cell body areas, and cortical and limbic structures using quantitative autoradiography. Treatment of rats with fluoxetine, but not amitriptyline, resulted in an attenuation of 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding in the dorsal and median raphe nuclei. The binding of the antagonist radioligand [3H]MPPF to 5-HT(1A) receptor sites was not altered, suggesting that the observed changes in 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding were not due to changes in receptor number. Thus, the desensitization of somatodendritic 5-HT(1A) autoreceptors in the dorsal and median raphe following chronic SSRI treatment appears to be due to a reduced capacity of the 5-HT(1A) receptor to activate G protein. By contrast, no significant change in postsynaptic 5-HT(1A) receptor-stimulated [(35)S]GTPgammaS binding was observed in any of the forebrain areas examined following chronic antidepressant treatment. Thus, changes in postsynaptic 5-HT(1A) receptor-mediated responses reported to follow chronic SSRI or tricyclic antidepressant administration most likely occur distal to receptor-G protein interaction, perhaps at the level of effector, or involving changes in neuronal function at the system or circuit level.

Mechanisms of ligand-induced desensitization of the 5-hydroxytryptamine(2A) receptor

We have examined the cellular processes underlying the desensitization of the 5-hydroxytryptamine (5-HT)(2A) receptor induced by agonist or antagonist exposure. Treatment of C6 glioma cells with either 5-HT or the 5-HT(2A) receptor antagonist ketanserin resulted in an attenuation in 5-HT(2A) receptor function, specifically the accumulation of inositol phosphates stimulated by the partial agonist quipazine. 5-HT-induced desensitization of the 5-HT(2A) receptor involved receptor internalization through a clathrin- and dynamin-dependent process because it was prevented by concanavalin A, monodansylcadaverine, and by expression of the dominant negative mutants beta-arrestin (319-418) and dynamin K44A. Although short-term (i.e., 10 min) 5-HT and ketanserin exposure resulted in the same degree of desensitization, ketanserin-induced desensitization was not prevented by these agents and did not involve receptor internalization. In contrast, prolonged ketanserin exposure (i.e., 2 h) resulted in 5-HT(2A) receptor internalization through a clathrin- and dynamin-dependent process, as was observed after agonist treatment. Inhibitors of protein kinase C or calcium-calmodulin kinase II did not attenuate or prevent 5-HT-induced desensitization of the receptor. 5-HT(2A) receptor desensitization induced by 5-HT and prolonged ketanserin treatment, but not by short-term ketanserin treatment, was prevented by the expression of the dominant negative mutant of G protein-coupled receptor kinase (GRK)2, GRK2-K220R, and by an anti-GRK2/3 antibody. Our data indicate a dual mechanism of early and late desensitization by the antagonist ketanserin. Short-term ketanserin treatment reduced the specific binding of the agonist radioligand [(125)I](+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ([(125)I]DOI) and the ability of 5'-guanylylimidodiphosphate to attenuate this binding, suggesting that at the early stage of antagonist-induced desensitization the capacity of the 5-HT(2A) receptor to couple to G protein is impaired.

The role of protein kinase C in the regulation of serotonin-2A receptor expression

We have investigated in C6 glioma cells the involvement of protein kinase C (PKC) in the regulation of serotonin-(2A) receptor (5-HT(2A) receptor) expression by agonist treatment. Comparison of the time-courses of agonist-induced downregulation of receptor number and mRNA indicate that a decrease in the number of 5-HT(2A) receptor binding sites in response to serotonin (5-HT) treatment is preceded by a decrease in 5-HT(2A) receptor mRNA. This decrease in 5-HT(2A) receptor mRNA as a result of agonist exposure was not due to a change in the stability or half-life of the transcript. Pretreatment of cells with the PKC inhibitor bisindolylmaleimide blocked the decrease in 5-HT(2A) receptor mRNA levels, and attenuated the down-regulation of 5-HT(2A) receptor binding sites induced by treatment with 5-HT. Experiments performed with the PKC inhibitors calphostin C and Gö 6976 confirmed that PKC was involved in the regulation of 5-HT(2A) receptor mRNA by agonist and implicate the conventional subgroup of PKC isoforms. Western blot analysis, using isoform-specific anti-PKC antibodies showed that under our culture conditions C6 glioma cells express the conventional isoforms PKC alpha, PKC gamma, as well as the novel isoforms PKC delta, PKC epsilon, and the atypical isoforms PKC lambda and PKC iota. Upon treatment with 5-HT for 10 min levels of the conventional isoforms PKC alpha and PKC gamma increased in the nuclear fraction. Taken together, our results implicate PKC alpha and/or PKC gamma in the regulation of 5-HT(2A) mRNA receptor and binding sites in response to agonist treatment.

gamma-Aminobutyric acid (GABA)--A function and binding in the paraventricular nucleus of the hypothalamus in chronic renal-wrap hypertension

The goal of this study was to determine whether gamma-aminobutyric acid (GABA)ergic transmission and GABA binding are altered in chronic renal-wrap hypertension. Three groups of hypertensive and sham-operated rats were prepared for separate protocols. Four weeks later, the animals were prepared with femoral artery catheters for the measurement of mean arterial pressure. In all groups, blood pressure was significantly higher in the renal-wrapped animals. In the first study, bilateral microinjection of the GABA-A antagonist, bicuculline (50 pmol/site), into the paraventricular nucleus of the hypothalamus (PVN) caused a greater increase in arterial pressure (21.9+/-1.4 versus 16.7+/-1.8 mm Hg, P<0.05) and heart rate (135+/-15 versus 98+/-12 bpm, P=0.064) in hypertensive rats. [(3)H]Flunitrazepam was used to measure binding to the GABA-A receptor. Magnocellular neurons and the adjacent medial parvicellular neurons had more intense binding compared with the remainder of the PVN. B(max) was greater for the higher density binding area; the K(d) value was less in the high-density region. There were no differences in these parameters between normotensive and hypertensive animals. Competitive reverse transcription-polymerase chain reaction was used to measure the expression of mRNA for the alpha(1) subunit of the GABA-A receptor. No difference was observed in the mRNA between renal-wrapped and sham-operated rats. In summary, inhibition of GABA-A receptors in the PVN is augmented in the chronic phase of hypertension and is unrelated to a change in the expression of the number or affinity to the receptor. These findings suggest that the greater GABAergic activity is the result of an increase in GABA release in the PVN in chronic renal-wrap hypertension.

Regulation of 5-HT(2A) receptor mRNA levels and binding sites in rat frontal cortex by the agonist DOI and the antagonist mianserin

In the present study we have characterized the time course of effect of administration of the serotonin(2) (5-HT(2)) receptor antagonist mianserin, or the 5-HT(2) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI), on 5-HT(2A) receptor binding sites and mRNA levels in rat frontal cortex. Radioligand binding and ribonuclease protection assays were performed with separate hemispheres of frontal cortex from each animal to examine concomitant changes in 5-HT(2A) receptor sites and mRNA levels. The decrease in cortical 5-HT(2A) receptor sites in response to chronic DOI administration was not accompanied by changes in 5-HT(2A) receptor mRNA. A single injection of DOI produced a transient decrease in 5-HT(2A) receptor mRNA levels detected 1 h post-injection. The density of 5-HT(2A) receptor sites, however, was not significantly reduced following a single injection of DOI. The down-regulation of cortical 5-HT(2A) receptor sites in response to a single injection of mianserin was accompanied by reductions in 5-HT(2A) receptor mRNA levels. Following 4 days of mianserin administration, however, we did not observe a change in 5-HT(2A) receptor mRNA levels, although 5-HT(2A) receptor density was decreased. Thus, changes in receptor mRNA may initially contribute to the down-regulation of 5-HT(2A) receptors in response to acute mianserin administration. Sustained changes in 5-HT(2A) receptor mRNA, however, appear not to be involved in maintaining the down-regulation of 5-HT(2A) receptor number with chronic mianserin administration. Mechanisms other than the regulation of receptor mRNA levels appear to underlie the down-regulation of 5-HT(2A) receptor sites in response to chronic administration of the agonist DOI.

Serotonin2A receptor modulation of D1 dopamine receptor-mediated grooming behavior

We have previously observed that intracerebroventricular infusion of a 5-HT2A receptor antisense oligonucleotide for 8 days results in an increase in cortical 5-HT2A receptor sites and an increase in central 5-HT2A receptor function as measured by quantitation of 5-HT2A receptor-mediated headshake behavior (28). Because lesioning serotonergic neurons or chronic administration of 5-HT2A receptor antagonists does not result in an increase in 5-HT2A receptor density or function in the brain, we have taken advantage of this unique upregulation of 5-HT2A receptors following 5-HT2A receptor antisense oligonucleotide infusion to study the modulation of D1 receptor-mediated behaviors by 5-HT2A receptors. Grooming behavior, elicited by acute injection of SKF 38393, was attenuated after chronic ICV infusion of a 5-HT2A receptor antisense oligonucleotide. There was also a reduction in vacuous chewing behavior induced by SKF 38393, which did not reach statistical significance. Other oral behaviors (i.e., tongue protrusions and gnawing at the cage bottom) were not attenuated. An increase in the density of cortical, as well as striatal 5-HT2A receptor sites was observed following chronic antisense oligonucleotide administration. There was no change in striatal D1 dopamine receptors following 5-HT2A receptor antisense oligonucleotide administration. SKF 38393-induced grooming behavior was also attenuated in naive rats pretreated acutely with the 5-HT2 receptor agonist DOI. These results suggest a role for the 5-HT2A receptor in the modulation of D1 receptor function.

Effect of chronic serotonin-2 receptor agonist or antagonist administration on serotonin-1A receptor sensitivity

We have investigated the effect of 5-HT2 receptor agonist or antagonist administration on postsynaptic 5-HT1A receptor sensitivity assessed by two behavioral measures, reciprocal forepaw treading or hypothermia induced by acute injection of the 5-HT1A receptor agonist 8-OH-DPAT. The effectiveness of these drug treatments to downregulate 5-HT2A receptors was confirmed by measuring the binding of [3H]-ketanserin in cortical homogenates, because all of these drug treatments have been shown to result in the downregulation of 5-HT2A receptor sites. Acute or chronic treatment of rats with the 5-HT2 receptor antagonist mianserin, or chronic administration of the 5-HT2A receptor antagonist ketanserin, did not alter 8-OH-DPAT-induced hypothermia or forepaw treading. These data indicate that downregulation of 5-HT2A receptors is not sufficient to alter these postsynaptic 5-HT1A receptor-mediated responses. Chronic treatment of rats with the 5-HT2 receptor agonist DOI, however, resulted in the attenuation of both 5-HT1A receptor-mediated responses measured in separate experimental groups. The apparent desensitization of 5-HT1A receptors following chronic DOI treatment was not accompanied by a change in either the number or affinity of 5-HT1A receptor sites as measured by the binding of [3H]-8-OH-DPAT in hippocampal homogenates. Chronic activation of 5-HT2 receptors may be one mechanism by which the sensitivity postsynaptic 5-HT1A receptors can be regulated.

Regulation of serotonin2A receptor expression by an antisense oligodeoxynucleotide

The regulation of 5-HT2A receptor expression by an antisense oligodeoxynucleotide, complementary to the coding region of rat 5-HT2A receptor mRNA, was examined in a cortically derived cell line and in rat brain. Treatment of A1A1 variant cells, which express the 5-HT2A receptor coupled to the stimulation of phosphatidylinositol (PI) hydrolysis, with antisense oligodeoxynucleotide decreased the maximal stimulation of PI hydrolysis by the partial agonist quipazine and the number of 5-HT2A receptor sites as measured by the binding of 2-[125I]-iodolysergic acid diethylamide. Treatment of cells with random, sense, or mismatch oligodeoxynucleotide did not alter the stimulation of PI hydrolysis by quipazine or 5-HT2A receptor number. Intracerebroventricular infusion of antisense, but not mismatch, oligodeoxynucleotide for 8 days resulted in a significant increase in cortical 5-HT2A receptor density and an increase in headshake behavior induced by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane. The density of cortical 5-HT2A receptors was not altered by administration of antisense oligodeoxynucleotide for 1, 2, or 4 days. We hypothesize that in brain this antisense oligodeoxynucleotide relieved some form of translational suppression, resulting in an increase in 5-HT2A receptor expression.

Expression and modulation of 5-hydroxytryptamine1A receptors in P11 cells

P11 cells were transfected with DNA for the human 5-hydroxytryptamine1A (5-HT1A) receptor. These cells stably expressed the 5-HT1A receptor coupled to the inhibition of adenylyl cyclase, and not to the stimulation of phosphoinositide hydrolysis. Homologous and heterologous regulation of the 5-HT1A receptor was studied in this cell system. Pretreatment of P11-5HT1A cells with the 5-HT1 receptor agonist 5-carbox-amidotryptamine (5-CT) resulted in a 3-fold increase in both basal and forskolin-stimulated cAMP accumulation, and desensitization of the 5-HT1A receptor as indicated by a decrease in the potency of 8-hydroxydipropylaminotetralin (8-OH-DPAT) to inhibit forskolin-stimulated cAMP accumulation (vehicle-treated cells: EC50 = 2.3 +/- 0.8 nM; 5-CT-treated cells: 9.9 +/- 0.4 nM). The sensitization of adenylyl cyclase as a result of chronic agonist exposure was prevented by the 5-HT1A antagonist WAY100635, which indicated that the effect of 5-CT pretreatment on basal and forskolin-stimulated cAMP accumulation was mediated by 5-HT1A receptor activation. Pretreatment of cells with pertussis toxin abolished the inhibition of forskolin-stimulated cAMP accumulation mediated by 5-HT1A receptor activation and prevented the sensitization of adenylyl cyclase as a result of chronic 5-HT1A receptor agonist exposure. Pretreatment of P11-5HT1A cells with the phorbol ester, phorbol 12-myristate 13-acetate (PMA), also resulted in desensitization of the 5-HT1A receptor, as indicated by a marked decrease in the potency and intrinsic activity of 8-OH-DPAT. No change in the binding characteristics (i.e., Kd or Bmax) of [3H]8-OH-DPAT to 5-HT1A receptor sites was observed after 5-CT or PMA treatments. Activation of alpha-1 adrenergic receptors, but not 5-HT2A receptors, had effects on 5-HT1A receptor responsiveness similar to those seen with PMA pretreatment. In P11-5HT1A cells, homologous regulation of the 5-HT1A receptor was characterized by sensitization of adenylyl cyclase and a decrease in agonist potency, whereas heterologous regulation of the 5-HT1A receptor was characterized by a greater decrease in agonist potency, as well as a marked decrease in intrinsic activity.

Quantitative autoradiography of the serotonin transporter to assess the distribution of serotonergic projections from the dorsal raphe nucleus

The binding of 3H-CN-IMI to 5-HT uptake sites, as measured by quantitative autoradiography, was used as a marker of serotonergic neurons. Within the dorsal raphe nucleus the binding of 3H-CN-IMI was compared in adjacent coronal sections of rat brain to the binding of 3H-DPAT to 5-HT1A receptors, which have a known somatodendritic localization. The heterogeneous pattern of binding of these two radioligands within the dorsal raphe nucleus was similar and corresponded to the distribution of serotonergic cell bodies as visualized by 5-HT immunohistochemistry. Intracerebroventricular administration of 5,7-dihydroxytryptamine (5,7-DHT), which caused a dramatic loss of 5-HT immunoreactivity and 3H-DPAT binding to 5-HT1A receptors, resulted in a marked reduction of 3H-CN-IMI binding in this nucleus. Treatment of rats with a dose of para-chloroamphetamine (PCA) which has been reported to selectively lesion serotonergic processes arising from the dorsal raphe nucleus, while sparing serotonergic cell bodies and projections from the median raphe nucleus, did not alter the binding of 3H-DPAT or 3H-CN-IMI in the dorsal raphe nucleus; serotonergic cell bodies appeared morphologically unaffected. The lack of effect of PCA treatment on the binding of 3H-DPAT and 3H-CN-IMI is consistent with a somatodendritic localization of the 5-HT transporter in the dorsal raphe nucleus. PCA treatment appeared to produce a moderate loss of serotonergic innervation in serotonergic terminal field areas as visualized by serotonin immunohistochemistry. The reductions in 3H-CN-IMI binding observed in terminal field areas (24 to 69%) following treatment of rats with PCA did not reflect a marked differential innervation of forebrain areas by the dorsal and medial raphe nuclei as expected from previous biochemical studies, and were not entirely consistent with the findings of neuroanatomical studies using histochemical techniques. Site-specific injection of 5,7-DHT into the dorsal raphe nucleus produced an 80 +/- 11% reduction in the binding of 3H-CN-IMI in this nucleus, whereas the binding of 3H-CN-IMI in the median raphe nucleus was not reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of [125I](R)-trans-7-hydroxy-2-[N-propyl-N-(3'-iodo-2'-propenyl)amino] tetralin binding to dopamine D3 receptors in rat olfactory tubercle

[125I](R,S)-trans-7-Hydroxy-2-[N-propyl-N-(3'-iodo-2'-propenyl)- amino]tetralin ([125](R,S)-trans-7-OH-PIPAT) has been shown to bind with high affinity to dopamine D3 receptors expressed in Spodoptera frugiperda cells. No specific binding was seen in Spodoptera frugiperda cells expressing a high density of D2 receptors. It was therefore, suggested that [125I] (R,S)-trans-7-OH-PIPAT selectively labels D3 receptors. In the present study, saturation binding of [125I](R)-trans-7-OH-PIPAT to membranes from rat olfactory tubercle resulted in markedly curvilinear Scatchard plots, suggesting that the radioligand was binding to more than one receptor class or affinity state. [125I] (R)-trans-7-OH-PIPAT bound with high affinity to membranes from human embryonic kidney-293 cells expressing transfected D2 or D3 receptors and to membranes from Chinese hamster ovary cells expressing serotonin1A receptors. Binding of [125I](R)-trans-7-OH-PIPAT to serotonin1A and D2 receptors was decreased or eliminated in the presence of NaCl and/or guanylyl-imidodiphosphate [Gpp(NH)p]. In the presence of Gpp(NH)p and NaCl, a linear Scatchard plot with a Kd value of 0.4 nM and a density of 100 fmol/mg of protein was obtained in membranes from rat olfactory tubercle. Agonists and antagonists inhibited binding of [125I](R)-trans-7-OH-PIPAT with a rank order of potency consistent with an interaction at D3 receptors. These results suggest that, in the presence of Gpp(NH)p and NaCl, [125I](R)-trans-7-OH-PIPAT specifically labels D3 receptors.